1. Field of the Invention
The present invention relate to an image forming lens suitable for reading an image of a document in an image reading device such as an image scanner or an image forming device such as a facsimile machine or a digital copier as well as to an image reading device and an image forming device incorporating such an image forming lens.
2. Description of the Related Art
An image reading device is provided in an image scanner, a facsimile machine or a digital copier for reading an image of a document. In the device an image of the document is reduced in size with an image forming lens, a reduced optical image is captured on a solid image sensor such as CCD (charge coupled device), and image information is converted into electric image data. To read document information in color, a three-line CCD image sensor in which light receiving elements with red, green, and blue filters are arranged in three lines is used to form a reduced optical image on a light receiving surface and separate the color of the image into 3 primary colors. Thereby, color image information is converted into signals. Such an image reading lens is required to have high contrast in a high spatial frequency domain on an image plane and high aperture efficiency, as high as near 100% to an image periphery. Also, to read color documents properly, it is necessary to align imaging positions of the three primary colors along an optical axis on the light receiving surface and accurately correct respective color aberrations. A widely-used Gaussian image reading lens is a relatively large-aperture lens which can realize high resolution. However, for the purpose of achieving currently desired high performance, such a Gaussian lens structure has a problem that it requires a large number of lenses of 6 or more such as four groups of six lenses or five groups of eight lenses.
In view of this, Japanese Patent No. 3939908 discloses an image reading lens comprising three to five lenses less than lenses of a Gaussian lens and exerting similar or higher optical performance. This image reading lens can be comprised of 3 lenses minimally and a lens closest to an image and disposed adjacent to an image plane is of a negative power. Thereby, it can easily correct various kinds of aberrations and achieve high performance.
However, the above reference fails to address a problem that lens performance is deteriorated during a thermal increase inside an image reading device due to an increase in heat emission amount of an imaging unit as CCD or of a high-illuminance light source along with downsizing and high speed operation of the image reading device. A thermal increase inside the device causes a thermal expansion and a change in refractive index of materials of an image reading lens, resulting in varying the focal length of the image reading lens. Further, the heat emission of the imaging unit brings a thermal expansion of a holder for the imaging unit and the image reading lens, shifting a positional relation between a lens imaging position and the imaging plane and deteriorating lens performance. Further, the above reference discloses only a lens with angle of view of up to about 45 degrees and fails to fully consider downsizing the image reading device.